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I'm reading about the crystals and found that the crystal can be operated in series mode resonance as well as parallel mode resonance.

In the case of series mode resonance, no external load capacitors are needed to be added to the crystal ends.

In the case of parallel mode resonance, there needs to external parallel load capacitors added to the crystal ends.

But in many designs I see external load capacitors are added to the crystal which means they are operating at parallel mode resonance? Any specific reason for it because in series mode, we can avoid the addition of capacitors right?

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  • \$\begingroup\$ Operating a crystal in the "wrong" mode will change its frequency by quite a large amount (1000ppm or so). So if you use a crystal in the series mode, make sure it's cut for the correct frequency in series mode. Better suppliers will do that for you; catalog crystals not so much. \$\endgroup\$
    – user16324
    Commented May 25, 2022 at 11:15

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Why can't we use crystal oscillators in series mode itself instead of parallel mode

Most crystal oscillator circuits are the "Pierce" type and they operate at a frequency between series and parallel crystal resonance (usually closer to series resonance but, very importantly NOT at series resonance).

I'm reading about the crystals and found that the crystal can be operated in series mode resonance as well as parallel mode resonance.

In a Pierce oscillator, crystals are never operated at exactly series resonance because the phase shift they will produce will be insufficient to make oscillation. In a Pierce oscillator, the operating frequency is slightly higher than the crystal's series resonance and slightly lower than its parallel resonance. Modified image from my basic website: -

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In the case of series mode resonance, no external load capacitors are needed to be added to the crystal ends

Well, your title is about crystal oscillators and, as explained in the previous paragraph, the most common type of oscillator is a Pierce circuit and it needs two capacitors to ensure a stable oscillation frequency.

But in many designs I see external load capacitors are added to the crystal which means they are operating at parallel mode resonance?

It is likely that the designs you have come across are Pierce oscillators (probably about +99% of all crystal oscillator circuits used) and, they operate slightly above series resonance and slightly below parallel resonance. They need capacitors to ensure they work correctly and stably.

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    \$\begingroup\$ Not to mention, as the load capacitance affects the frequency of oscillation, adjusting the capacitor values allow the frequency to be adjusted. And if the crystal requires a relatively large load capacitance, it means that the frequency does not change much if for some reason the stray capacitance has small variations. \$\endgroup\$
    – Justme
    Commented May 25, 2022 at 10:12
  • \$\begingroup\$ Thank you for the answer. \$\endgroup\$
    – user220456
    Commented May 25, 2022 at 10:15

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